This year’s Waterstones children’s book prize winner features a green blob tucking its child – a smaller green blob – into bed.

Sleep Tight, Disgusting Blob by Huw Aaron is narrated by the parent blob who, through the course of the book, tells the child blob about all the other creatures also getting ready to sleep, including a yeti, Medusa and a Minotaur.

Aaron, a Welsh author and illustrator, was announced as the winner of the £5,000 prize at a ceremony on Thursday evening. His book topped the illustrated books category before going on to be named overall winner.

Booksellers, who vote for the prize, praised Huw’s “dreamy, painterly style and sweet rhyming text”, said Bea Carvalho, head of books at Waterstones.

Aaron is from Swansea, and now lives in Cardiff. Growing up, he spent much time doodling and writing stories, but trained as an accountant. “It shortly became apparent that drawing monsters and developing elaborate toilet-based puns were not the primary skills required of an NHS financial manager and, in 2009, Huw was belched out like Jonah of old from the whale of public-sector accountancy on to the beach of self-employment as a freelance cartoonist,” he writes on his website. He has created cartoons for magazines including Private Eye and Prospect.

Another book of Aaron’s, Unfairies, was nominated in the younger readers category, marking the first time in the award’s history that an author has been shortlisted for two titles simultaneously.

Ultimately, Janeen Hayat went on to win the younger readers category, with her book Evie and Maryam’s Family Tree, in which two classmates discover they have a family connection dating back to the partition of India.

A Language of Dragons by SF Williamson was named the older readers winner. The book is set in an alternate Bletchley Park, featuring dragons, where the protagonist, Vivien Featherswallow, has been recruited as a codebreaker.

The Waterstones children’s prize was first awarded in 2005. Past recipients of the overall award include The Hate U Give by Angie Thomas, Rooftoppers by Katherine Rundell and Blown Away by Rob Biddulph. Last year’s prize was awarded to Mikey Please for The Cafe at the Edge of the Woods. Along with the cash prize, the winner is promised “ongoing commitment” to their writing and illustrating career.

The broadcaster Greg James has apologised after being criticised over a promotional video for his followup to the Roald Dahl novel The Twits, in which it was suggested giving one of the principal characters a glass eye would make her disgusting.

In the video, James appeared with his colleague Chris Smith and the illustrator Emily Jones, as she rendered the characters the other two have written about. They listed several traits they felt would make them more unpleasant, including the addition of the glass eye, which Mrs Twit had in the original novel.

“We are so sorry to have caused offence with the launch video,” James said. “It was absolutely not our intention. And we apologise unreservedly. It’s now gone. We understand that words matter and we pride ourselves on championing and welcoming everyone into the magical world of children’s books.

“It’s unfortunate that the word disgusting appears to be a direct comment on that,” James added. “That’s an error on our part. We are devastated that it’s come across like this but completely understand why there is anger.”

According to Mail Online, the Royal National Institute of Blind People had criticised the promotional video. “When there’s positive representation of disabilities in children’s books, a truly wonderful thing happens – children with disabilities feel seen and heard, and their friends and classmates treat everyone the same,” it said. “There is nothing at all revolting about prosthetic eyes, we think they’re brilliant.

“We strongly urge all authors, illustrators, or publishers to think about the influence they have and the role they can play in encouraging acceptance and understanding of disabilities, both visible and invisible, especially for children with vision impairments.”

The Twits and Matilda are among the Roald Dahl characters set to appear in brand new stories inspired by the late British author’s work, due to be published later this year.

Stories written by James, Adam Kay and Konnie Huq will celebrate Dahl’s “unique sense of the extraordinary, the magical and the mischievous”, said Ruth Knowles, the publishing director at Puffin.

James and Smith have co-written a story titled The Twits Next Door, illustrated by Jones, to be published in August. When the Lovelies move in next door to Mr and Mrs Twit, the Twits try to rid themselves of their “horribly nice” new neighbours.

Meanwhile, a festive collection, Charlie and the Christmas Factory, is planned for October, comprising 12 short stories based on the favourite Dahl characters of the collection’s authors. These include Huq, Kay, Adam Hills, Ben Bailey-Smith, Elle McNicoll, Sibéal Pounder and Nadia Shireen among others.

In McNicoll’s story, Matilda is worried about being on the naughty list, while Pounder looks at how Charlie plans to celebrate Christmas at the chocolate factory. In Kay’s tale, set years after the creation of George’s marvellous medicine, his granddaughter Gigi decides to make her own potion.

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“We have been so impressed by the authors’ brilliant ability to capture the unique spirit of a Roald Dahl story and bring us some wonderfully funny tales of their own imagination,” said Harriet Murphy, the publishing director at the Roald Dahl Story Company. “Roald Dahl’s stories are among some of the greatest children’s books, but we aren’t looking to imitate him. Instead, we wanted the authors to have fun and to use their own voices to take his extraordinary characters and creations into new stories.”

The collection will be illustrated by Rikin Parekh. “Of course, I am a firm believer in the magic that is created when words and illustrations blend perfectly,” he said. “And no combination is as magical as that of Roald Dahl and Quentin Blake.”

The first in a series of picture-book versions of Dahl’s Revolting Rhymes will be released in May, with illustrations by Adam Larkum.